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Organometallics 1996, 15, 4141-4146
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Synthesis and Characterization of Boron-Containing Ferrocenyl Ligands for Asymmetric Catalysis Barbara F. M. Kimmich, Clark R. Landis,* and Douglas R. Powell Department of Chemistry, University of WisconsinsMadison, 1101 University Avenue, Madison, Wisconsin 53706 Received June 4, 1996X
Novel boronato-functionalized ferrocenylphosphine ligands 3a-c have been synthesized from (S,R)-1,1′-bis(diphenylphosphino)-2-[1((1-hydroxy-2-phenyl)amino)ethyl]-ferrocene (2). These ligands form stable ligand-metal adducts with Rh(I) complexes that are catalytically active in hydrogenation and hydroformylation reactions. The square-planar metal complex Pt(2)(Me)I exhibits a structure that appears well-suited to support secondary interactions between the Lewis acidic boron functionality and Lewis basic sites of functionalized, coordinated alkenes. Introduction Tremendous progress has been made in the development of highly selective and active homogeneous transition-metal catalysts.1 Many catalysts, e.g. those that employ chiral bidentate ligands such as BINAP2 and DuPHOS,3 impart enantioselectivity through steric interactions between a substrate and the coordinated ligand. Recently, Ito has coined the term “secondary interactions” to describe catalysts that control selectivity via weak bonding interactions between the substrate and the ligand.4 These attractive catalyst-substrate interactions occur outside the metal’s primary coordination sphere and may include electrostatic interactions,5 hydrogen bonding,6 and Lewis acid-base interactions.7 The identification of ligand structures that can predictably place functional groups in the proximity of their substrate-based complements is critical to developing the control of selectivity via secondary interactions. In this paper we report progress in the construction of ligand frameworks that may lead to catalytically useful secondary interactions. Two criteria to be satisfied in designing ligands that impart selectivity via secondary interactions are (1) the framework of the ligand must allow secondary interactions without prohibitive strain, leading to a reaction transition state which is stabilized with respect to undesired reaction pathways, and (2) the secondary interaction must be formed rapidly and reversibly. Our tactile and computer-generated modeling studies indicate that the ferrocenyl bis(phosphine) backbone provides a rigid framework which can orient the secondary X Abstract published in Advance ACS Abstracts, September 1, 1996. (1) For reviews see: (a) Ojima, L.; Clos, N.; Bastos, C. Tetrahedron 1989, 45, 6901. (b) Noyori, R.; Kitamura, M. In Modern Synthetic Methods; Scheffold, R., Ed.; Springer-Verlag: Berlin, 1989; Vol. 5, p 115. (2) Miijashita, K.; Yasuda, A.; Takaya, H.; Toriumi, K.; Ito, T.; Souchi, T.; Noyori, R. J. Am. Chem. Soc. 1982, 102, 7932. (3) Burk, M. J.; Feaster, J. E.; Harlow, R. L. Organometallics 1990, 9, 2653. (4) Sawamura, M.; Ito, Y. Chem. Rev. 1992, 92, 857. (5) Sawamura, M.; Nagata, H.; Sakamoto, H.; Ito, Y. J. Am. Chem. Soc. 1992, 114, 2586. (6) Hayashi, T.; Kanehira, K.; Hagihara, T.; Kumada, M. J. Org. Chem. 1988, 53, 113. (7) (a) Bo¨rner, A.; Ward, J.; Kortus, K.; Kagan, H. B. Tetrahedron. Asymmetry 1993, 4, 2219. (b) Fields, L. B.; Jacobsen, E. N. Tetrahedron. Asymmetry 1993, 4, 2229.
Figure 1. (a) Proposed Lewis acid-base secondary interaction. (b) Proposed covalent secondary interaction.
interaction over an open coordination site of a catalytic metal center.8 This widely used class of ligands is derivatized readily, making possible a wide variety of different secondary interactions.6,9 In this paper we focus on the attachment of boronato groups to the diphosphine ligands. These functional groups are weak Lewis acids that are known to reversibly bind Lewis bases such as amines (Figure 1a).10 Additionally, borates and aminoborates rapidly transesterify and transamidify with alcohols and amines,11 respectively, making them prime candidates for covalent secondary interactions (Figure 1b). Herein we report the preparation, spectroscopic and structural characterization, and catalytic properties of metal complexes of the novel boron-containing ligands 3a-c. Results and Discussion (S,R)-1,1′-Bis(diphenylphosphino)-2-(1-acetoxyethyl)ferrocene (1; (S,R)-BPPFAc) is a useful starting material for making chiral substituted bis(phosphine) ferrocenyl ligands.9 By a procedure similar to that used by Ito and co-workers9 (S,R)-1,1′-bis(diphenylphosphino)-2-[1-((1hydroxy-2-phenyl)amino)ethyl]ferrocene (2; (S,R)-BPPFAP) formed when 1 was refluxed in toluene with a large excess of 2-aminophenol for 18 h (Scheme 1). The (8) Cleveland, T.; Landis, C. R. Unpublished results. (9) Hayashi, T.; Mise, T.; Fukushima, M.; Kagotani, M.; Nagashima, N.; Hamada, Y.; Matsumoto, A.; Kawakami, S.; Konishi, M.; Yamamoto, K.; Kumada, M. Bull. Chem. Soc. Jpn. 1980, 53, 1138 and references cited therein. (10) (a) Toyota, S; Oki, M. Bull. Chem. Soc. Jpn. 1990, 63, 1168. (b) Narasaka, K.; Sakurai, H.; Kato, T.; Iwasawa, N. Chem. Lett. 1990, 1271. (11) Brauch, T. W.; Landis, C. R. Unpublished results.
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Organometallics, Vol. 15, No. 20, 1996
Figure 2. X-ray crystal structure for rac-BPPFAP (2). The displacement ellipsoids are drawn at 35% probability. Hydrogens have been removed for clarity. Selected bond lengths (Å) and bond angles (deg): N(11)-C(11), 1.454(10); N(11)-C(13), 1.389(10); O(14)-C(14), 1.385(9); N(11)C(13)-C(14), 118.5(7); O(14)-C(14)-C(13), 114.9(8).
Kimmich et al.
Figure 3. X-ray crystal structure for rac-BPPFAP-BMe (3a). The displacement ellipsoids are drawn at 50% probability. The solvent molecule and hydrogens have been removed for clarity. Table 1. Selected Bond Lengths (Å) and Angles (deg) for rac-BPPFAP-BMe (3a)
Scheme 1 C(23)-N(25) N(25)-C(34) N(25)-B(26) C(34)-N(25)-C(23) C(34)-N(25)-B(26) B(26)-N(25)-C(23) N(25)-B(26)-C(27)
Bond Lengths 1.486(5) B(26)-C(27) 1.403(5) B(26)-O(28) 1.428(6) O(28)-C(29) Bond Angles 123.3(3) O(28)-B(26)-N(25) 107.4(3) O(28)-N(25)-C(27) 129.5(4) C(29)-O(28)-B(26) 130.2(4)
1.564(7) 1.415(6) 1.380(5) 108.0(4) 121.9(4) 107.2(3)
Scheme 2
1H
and 31P NMR and liquid secondary ion mass spectrometry (LSIMS) spectra and the X-ray crystallographic structure (Figure 2) are all consistent with the formulation of (S,R)-BPPFAP. The overall structural characteristics of BPPFAP are similar to those of 1-[(R)1′,2-bis(diphenylphosphino)ferrocenyl]-(S)-N,N-dimethylethylamine.12 (S,R)-BPPFAP (2) was reacted with a series of boric acid derivatives using a procedure based on the work of Fields and Jacobsen (Scheme 1).7b The resulting boron-containing ferrocenyl ligands (S,R)-3a-c (BPPFAP-BMe, BPPFAP-BPh, and BPPFAP-BPhCl, respectively) as solids were handled in air with no apparent decomposition. The crystal structure of rac-3a is shown in Figure 3 (see Table 1 for selected bond lengths and angles). Ligands 2 and 3a-c form stable metal complexes. Reaction of the boronate ligands 3a-c with PtMe2(COD) (COD ) cyclooctadiene) in benzene at 60 °C yielded the complexes 4a-c (Scheme 2). Formation of the platinum complexes was supported by the characteristic Pt coupling pattern in 31P NMR spectra,13 the appearance of inequivalent Pt-Me and free cyclooctadiene resonances in 1H NMR spectra, and the mass spectral data. LSIMS spectra reveal weak peaks corresponding to parent ions (12) Shawkataly, O. B.; Fun, H.-K.; Chinnakali, K.; Yip, B.-C.; Teoh, S.-G.; Ito, Y.; Sawamura, M. Acta Crystallogr. 1993, C49, 139. (13) 195Pt is 33.8% abundant (I ) 1/2) and yields a characteristic satellite pattern which looks like a triplet.
of 4b and 4c and strong peaks for ions generated by the loss of one methyl group for 4a-c. rac-BPPFAP (2) reacted with PtMe2(COD) to give a single product which exhibits 31P NMR resonances similar to those for complexes 4a-c. However, this complex slowly decomposed in solution, presumably via methane loss due to protonolysis of the Pt-Me bond by the weakly acidic phenol. The reaction of rac-BPPFAP with PtMeI(COD) yielded the stable complex PtMeI(2), which is far less susceptible to loss of the methyl group than PtMe2(BPPFAP) (Scheme 3). Because the phosphines are inequivalent, two geometric isomers are possible. In solution both isomers, 5a and 5b, are present approximately in a 1:1 ratio, as revealed by NMR spectra. The crystal characterized by X-ray crystallography comprised a mixture of isomers (Figure 4, Table 2) in a 30:70 ratio (5a:5b). The two phosphorus atoms coordinate to platinum, occupying cis sites in a distorted-square-planar geometry. The environment of
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Organometallics, Vol. 15, No. 20, 1996 4143 Table 3. Hydrogenation of Methyl (Z)-r-Acetamidocinnamate (MAC)
Figure 4. X-ray crystal structure for PtMeI(rac-BPPFAP) (5b). The displacement ellipsoids are drawn at 50% probability. The acetone molecule and all hydrogens have been removed for clarity. The methyl and iodide groups attached to the platinum were disordered. Isomer 5b had an occupancy of 0.7299(12).
ligand
initial turnover freq (turnovers/min)
% ee
1 2 3a 3b 3c
120 138 149 22 127
9 54 39 49 53
modeling using the UFF force field suggested that the overall ligand structure is hardly perturbed upon attachment of boron to the aminophenol. Furthermore, modeling studies suggested that such a structure is capable of supporting either Lewis acid-base or covalent bonding between the boron and allylamine or allyl alcohol, respectively. It is interesting to note that 5a shares some of the structural characteristics of [Pd(η3-allyl)(6)]ClO4, a highly selective catalyst for asymmetric allylic amination.15 In
Scheme 3
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for PtMeI(rac-BPPFAP) (5b) Pt(1)-C(1′) Pt(1)-I(1) P(1)-Pt(1) P(2)-Pt(1) N(13)-C(11) (C1′)-Pt(1)-I(1) P(1)-Pt(1)-P(2) P(1)-Pt(1)-I(1) P(2)-Pt(1)-C(1′)
Bond Lengths 2.163(7) N(13)-C(14) 2.5989(4) O(20)-C(15) 2.3215(10) I(1)-N(13) 2.2737(10) I(1)-O(20) 1.467(5) Bond Angles 85.8(2) N(13)-C(14)-C(15) 97.90(4) O(20)-C(15)-C(14) 92.80(3) I(1)-N(13)-Pt(1) 84.8(2) I(1)-O(20)- Pt(1)
1.384(6) 1.367(6) 4.008(2) 3.968(2)
116.7(4) 115.2(4) 69.9(6) 108.4(6)
the platinum and phosphorus atoms is quite similar to that of other bis(diphenylphosphino)ferrocene-platinum complexes.14 For example, PtCl2(dppf) (dppf ) bis(diphenylphosphino)ferrocene) has Pt-P bond lengths (2.252(4) and 2.260(4) Å) similar to those of complex 5b (2.3215(10) and 2.2737(10) Å).14a Significantly, the crystallographic structure of 5a reveals a proximal relationship between the aminophenol functional group and the metal coordination sites. The O and N atoms of the aminophenol each are located 4.0 Å (Table 2) above the coordinated iodide in a direction approximately perpendicular to the square plane about the Pt. Preliminary molecular mechanics (14) (a) Clemente, D. A.; Pilloni, G.; Corain, B.; Longato, B.; Tiripicchio-Camellini, M. Inorg. Chim. Acta 1986, 155, L9. (b) Bandoli, G.; Trovo´, G.; Dolmella, A.; Longato, B. Inorg. Chem. 1992, 31, 45. (c) Vasapollo, G.; Toniolo, L.; Cavinato, G.; Bigoli, F.; Lanfranchi, M.; Pellinghelli, A. J. Organomet. Chem. 1994, 481, 173.
the structure of [Pd(η3-allyl)(6)]ClO4, the pendant side chain on the ferrocenylphosphine ligand is directed toward the reaction site on palladium. The terminal hydroxyl group is located at a position close to one of the π-allyl carbon atoms (3.36(1) Å compared to a distance of 5.24(1) Å from the other carbon). This structural evidence supports Ito and co-workers’ proposal that the high selectivity of this catalyst correlates with interaction of the terminal hydroxyl and the incoming amine, thus directing the nucleophilic attack at one of the π-allyl carbons. Ligands 1, 2, and 3a reacted also with [Rh(NBD)2]OTf (NBD ) norbornadiene, OTf ) trifluoromethanesulfonate) in acetone to yield the corresponding (S,R)[Rh(NBD)(L)]OTf complexes, 7-9. These complexes were isolated as orange-red solids and were characterized by 1H NMR, 31P NMR, and LSIMS. Similar complexes are formed for ligands 3b,c, as indicated by 31P NMR. These complexes, however, could not be isolated as pure materials and slowly decomposed over time in solution into unknown species (t1/2 ≈ 3 days). Successful hydrogenation and hydroformylation of simple substrates established the catalytic competency of complexes containing the ligands 1-3. All five ligands catalyzed the hydrogenation of MAC, methyl (Z)-R-acetamidocinnamate (Table 3). For reasons that are unclear, ligand 3b yielded a significantly slower catalyst. In keeping with previous results for the hydrogenation of MAC catalyzed by Rh complexes of ferrocene-derived bis(phosphines), unremarkarble enantiomeric excesses (
